High strength aluminum extrusion alloy

ABSTRACT

High strength extrudable and readily weldable aluminum base alloys are prepared comprising 0.9-1.5% magnesium, 0.4-0.8% silicon, and 0.9-1.5% copper, which may also include optional elements such as manganese, iron, and chromium, wherein the silicon content must not exceed the sum of 0.58 × magnesium content plus 0.25 × the manganese plus iron contents and the copper content must not exceed the sum of magnesium plus silicon contents. Such alloys display improved retention of strength properties after being subjected to welding conditions.

BACKGROUND OF THE INVENTION

The present invention relates to high strength aluminum base alloys andparticularly to wrought high strength aluminum base alloys produced inextruded or hot-rolled plate form, which are well adapted for weldingoperations in further fabrication steps, wherein the strength propertiesare retained at high values, even exceeding about 40 ksi for the yieldstrength of extruded products and 30 ksi for hot rolled plate, withoutany necessity for interposing special heat treatment steps.

The alloy compositions in accordance with this invention have been shownto meet the specified requirements and have furthermore surprisinglyprovided excellent solutions to the problems and disadvantagesconsistently associated with previous attempts to use prior art alloycompositions for such purposes. Such attempts were accompanied byinordinate loss of strength properties on welding, and/or a requirementafter welding for special heat treatment and artificial aging steps torecover at least part of the lost strength properties, and/or anexcessive tendency to undergo weld failures, such as under-bead weldcracks, and/or susceptibility to various types of corrosion, such asstress corrosion or exfoliation corrosion, which might result inexcessive failures in service.

Thus, at least one of the foregoing disadvantages, and usually severalof them is encountered in attempts to weld previously knownhigh-strength aluminum base alloys which include magnesium, silicon andcopper as essential components, as occurs in such attempted use of AAAlloys 6066 and 6351, and of alloy compositions as disclosed in U.S.Pat. Nos. 3,498,221 and 3,935,007 and in British Pat. No. 1,383,895,also described in Journal of Metals (September, 1976), pages 15-18,which in general were formulated to accomplish purposes differing fromthe present objectives.

Accordingly, it has been a principal object of the present invention toprovide improved high strength aluminum base alloy compositionscharacterized by the capability of being welded readily withoutundergoing an excessive decrease in strength properties.

A further object has been the provision of such alloy compositionscharacterized by the capability of being formed by extrusion or byhot-rolling procedures.

Another object has been the provision of such alloy compositionscomprising a defined range of magnesium content in conjunction withother essential elements in proportions required to achieve the desiredfunctional characteristics.

A further object has been the provision of such alloys characterized byheat-treatability and natural aging characteristics.

Another object has been to provide such alloy compositions readilysuitable for conversion to wrought products.

Further objects and advantages of the present invention will be apparentfrom the following detailed description.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has now been found that theabove objects can be advantageously obtained by the provision of alloycompositions consisting essentially of 0.9-1.5% magnesium, 0.4-0.8%silicon, and 0.9-1.5% copper, wherein the copper must not exceed the sumof magnesium and silicon, and the silicon must not exceed the sum of0.58 × percent Mg + 0.25 × percent (Mn + Fe). One or more of the groupCr, Mn, and Fe, is usually present, particularly in extrusion alloys, ata content of about 0.05-0.4% and the balance, other than added elementsand usual impurities, is essentially aluminum. The added elements may beone or more of the following at the stated weight percentage ranges:0.01-0.2 zirconium, 0.01-0.2 titanium, 0.01-0.2 vanadium, 0.01-0.4cobalt, and 0.01-3.5 nickel. As will be discussed later, such additionalelements are beneficial in the strengthening and stabilization of thewrought structure induced by hot working, through the formation of finedispersed intermetallic precipitates. Other elements may be present asimpurities in percentages up to about 0.05% each and totalling less than0.15%, without adversely affecting the desired properties. In apreferred embodiment, the alloys of the present invention may contain1.0-1.5% Mg, 0.4-0.7% Si, 1.0-1.5% Cu, and 0.2-0.4% of one or moreadditive elements selected from the group consisting of Mn, Fe and Cr,and the balance essentially aluminum.

Alloys in accordance with this invention have enabled the attainment inarticles, after thermal treatments met in welding, of yield strengths ofover 30 or 40 ksi, without requiring processing other than naturalaging. This represents a major advance over prior art practices andaccomplishments, for example as summarized in Aluminum, Volume 3,American Society for Metals 1967), Chapter 12, especially, pages407-415. In contrast, temper-rolled sheets of Alloy 5456, the higheststrength composition in the non-heat-treatable 5000 series of aluminumalloys display a loss in strength properties after welding to values ofyield strength and tensile strength characteristic of annealed metal.While certain heat-treatable Al base alloys could be chosen whichdisplayed better retention of high strength values after welding, thesegave rise to other problems and disadvantages such as cracked orotherwise unsatisfactory welds, inadequate corrosion resistance, or theneed for special heat treatment procedures.

In order to facilitate a comprehensive study aimed at establishingimproved alloy compositions for this purpose, a simulated welding testwas developed which would accurately indicate the strength propertiesresulting on the application of the welding procedure. This wasaccomplished by forming a single pass edge weld on each face of twoplate halves 0.25 inch thick of 6061-T6 aluminum alloy, recordingtime-temperature curves for measured times up to 90 seconds and at aseries of distances on each side of the weld. Hardness, tensile strengthand yield strength values, and microstructure were determined for thesepoints. This study established that the effects of low energy(corresponding to single pass) MIG welding (by electric arc under inertgas, using filler wire of alloy 5356 at rates of 15 and 30 inches perminute) could be reproduced by immersing a plate of sample alloy, 0.060inch thick, in molten salt at 750° F. for 10 seconds and cooling instill air, and high energy welding (corresponding to multi-pass orrepair conditions) could be reproduced by treatment in molten salt at750° F. for 20 seconds.

The above simulated welding test was found to accomplish a loss inhardness and strength properties and a change in microstructurecorresponding to the changes determined to occur within a zone about 0.3to 0.4 inch from the weld bead centerline. Thus, the initial tensilestrength decreased from about 50 ksi to about 30 in the 10 secondtreatment and to about 25 in 20 seconds; the yield strength was loweredfrom 45 to about 20 in 10 seconds and to about 15 in 20 seconds. Thestudy of microstructure established that the above zone, within whichthe tensile fractures during strength evaluation tests occurred, wascharacteristic of an overaged region containing coarsened particles ofprecipitated Mg₂ Si. Neither the welded plates nor the samples treatedin molten salt displayed any natural aging after storage, that beingprecluded by the completeness of the precipitation during the treatment.

The availability of the above-described simulated welding test enabledthe completion of a series of screening tests of varied aluminum alloycompositions, the results of which indicated that the desired objectivesmight well be attainable through the enhancement ofaluminum-magnesium-silicon alloys by increasing their initial strengthproperties, while providing against undue loss of strength duringwelding, at the same time improving the resistance to over-aging, andthrough the simultaneous imparting of a natural aging response, whichwould occur after the welding operation. As substantiated in thefollowing specific examples, the objectives were attained by thecompositions specified herein, within the determined ranges of thestated proportions and with strict observance of the maximum permissiblelimit of silicon in proportion to the content of magnesium, iron andmanganese, and providing a copper content not in excess of the sum ofmagnesium plus silicon.

The stated composition limits, as established by a comprehensive seriesof experiments, basically are those which have been found to provide thedesired high strength and other essential properties, includingweldability without the undue loss of strength, and to displaysatisfactory resistance to stress corrosion and to corrosion by variousenvironments which might be encountered during use.

The effective range of magnesium content is such as to provide increasedinitial strength properties effected through the presence of finelydispersed Mg₂ Si particles, as well as adequate retention of suchproperties through the welding cycle. Such effects are not obtainedbelow the specified minimum content of Mg, while amounts of Mg exceedingthe maximum are disadvantageous in increasing the tendency towardoveraging during the welding treatment, with consequent undue losses instrength properties. Furthermore, the use of over 1.5% Mg in the alloyis disadvantageous, tending to effect a decreased resistance to stresscorrosion. However, an excess of Mg in relation to Si is preferred, astending to inhibit over-aging and to promote the recovery of strengththrough natural aging.

The useful range of copper was established as between 0.9 and 1.5% asthese proportions provided substantial increases in the initial strengthproperties, particularly in yield strength and tensile strength,increased the retention of strength during the welding operation, andimparted gains in strength through natural aging following welding.These features were not displayed to any substantial extent bycompositions containing less Cu than the minimum. At above the maximumof 1.5% Cu, the strength retention effect was less marked, and thetendency toward deteriorating effects due to environmental corrosion wasgenerally increased.

The above effects in the beneficial range appear to be brought about bythe introduction of additional phases and the substantially uniformdistribution of the fine hardening intermetallic precipitates throughoutthe metal. A synergistic effect thereof is the precipitation of Mg₂ Sias tiny needles or rods rather than as large plates or grains found tooccur in compositions containing insufficient proportions of copper.

The specified range for the optional added elements, particularlymanganese, iron, and chromium likewise states the limits within whichthe most effective initial strength increase and strength retentionduring welding are obtained, as the use of less than minimal proportionspresents no substantial benefit and the presence of proportions higherthan the maximum are correspondingly less effective and may introducedisadvantageous tendencies toward decreased corrosion resistance andimpaired natural aging benefits.

Similar effects exist with respect to departures from the specifiedrange of silicon content, where it is also critical to observe thelimitation that the Si content must not be more than corresponds to thesum of 0.58 × Mg content + 0.25 × content of (Mn + Fe). This limitationcorresponds to the provision of excess magnesium over that required tocombine with silicon to form precipitated silicide, which has beenindicated to produce the most advantageous combination of desiredproperties, particularly of high initial strength, retention of strengthduring welding, and increase in strength by natural aging following thewelding procedure. The presence of excess Si has been found to benotably disadvantageous with respect to the latter two of the abovefeatures. In contrast, the effect of excess magnesium is most evidentunder high energy welding conditions, where subsequent natural agingresults in the most significant recovery of strength properties.

DETAILED DESCRIPTION

Compositions in accordance with the invention and comparison alloys weremelted, fluxed by treatment with chlorine gas for 5 minutes or with anitrogen-dichlorodifluoromethane mixture for 10 minutes, and cast as 5pound Durville ingots, using a pouring temperature of 1320° F. Theingots, after homogenization at 930° F. for 24 hours, were cut into 4inch square sections, 0.75 inch in thickness. These sections were hotrolled at 930° F. in a single pass to a thickness of 0.15 inch and waterquenched. Such sections, requiring no solution treatment before aging,could be used to estimate the press quench effect which might beexpected in commercial scale extrusions.

A portion of the hot rolled plate was cold rolled to a thickness of0.060 inch, solution annealed, water quenched, and aged for 18 hours at320° F. to develop peak aging properties, denoted as -T6 temper.

Another portion of the above hot rolled plate was tested after beingaged for 18 hours at 320° F., denoted as -T5 temper.

Tests on Al alloyed with 0.36 to 1.0% Mg and 0.25 to 1.5% Si at -T6temper, prepared as described above, resulted in measured values ofyield strength (Y) -- tensile strength (T) -- elongation (E),respectively, of 12 ksi -- 18 ksi -- 13 initially for an alloy of 0.36%Mg, 0.25% Si, and balance Al, and 4 -- 13 -- 28 after immersion for 10seconds at 750° F. (simulated welding test). The corresponding valuesfor an alloy of 0.71 Mg, 1.5 Si, and balance Al were 40 -- 44 -- 6 and14 -- 22 -- 14, respectively. Ternary alloys of these elements inproportions between the above limits yielded intermediate values, withlosses after the welding test ranging from 8 to 26 ksi in yield strengthand from 5 to 22 ksi in tensile strength. Similar values of strengthlosses also resulted with similar alloys, each containing a smalladdition of Sn, Cd, Mn, Co, V, or Cr.

This series also included three comparison Al alloys containing Mg, Si,and Cu, in proportions not in accordance with the present invention,which yielded test results similar to the above, as shown in Table I.

                  TABLE I                                                         ______________________________________                                                                  After 10 Secs.                                                         Initial                                                                              At 750° F                                    Alloy Mg      Si      Cu    Al   Y T E  Y T E                                 ______________________________________                                        1     0.66%   0.44%   0.25% Bal. 35-39-12                                                                             15-21-13                              2     0.71    0.45    1.5   Bal  42-52-13                                                                             25-34-12                              3     0.75    0.47    3.1   Bal  49-58-0                                                                              30-43-10                              ______________________________________                                    

In contrast, the following examples will be seen to substantiate theattainment of the objectives of the present invention by the provisionof alloy compositions in accordance therewith.

EXAMPLE I

Alloy A, containing 1.38% Mg, 0.67% Si, 1.41% Cu, 0.39% Mn, balance Al(all percentages being by weight, unless otherwise indicated), tested at-T5 temper, displayed the following tensile properties initially, after10 seconds at 750° F., after 20 seconds at 750° F., and followingnatural aging for 2 weeks after each treatment, shown in Table II.

                  TABLE II                                                        ______________________________________                                                           Y     T       E                                            ______________________________________                                        Initial              41      56      15                                       After 10 Seconds at 750° F                                                                  33      45      13                                       Then, aged 2 weeks   37      48      14                                       After 20 Seconds at 750° F                                                                  26      39      14                                       Then, aged 2 weeks   33      45      14                                       ______________________________________                                    

Thus, the simulated low energy welding test caused a substantiallysmaller loss in tensile properties than resulted in the previous tests.Furthermore, natural aging following the high energy test (20 seconds)resulted in restoring much of the lost strength.

EXAMPLE II

Comparison alloys having the following compositions not in accordancewith the invention were subjected at -T5 temper to the same tests asused in the previous example.

                  TABLE III (a)                                                   ______________________________________                                        Alloy Mg      Si      Cu     Other       Al                                   ______________________________________                                        4     0.50%   1.03%   0.02%  .38 Fe, 0.49 Mn,                                                                          Bal.                                                              0.007 Ti, 0.043 Zn                               5     1.35    0.68    1.53   0.41 Mn     Bal.                                 6     1.35    0.74    0.54   0.42 Mn     Bal.                                 ______________________________________                                    

Test results were as follows:

                  TABLE III (b)                                                   ______________________________________                                        Tensile Properties (Y-T-E)                                                    Com-           After 10 Secs. After 20 Secs.                                  pari-          at 750° F                                                                             at 750° F                                son                      Aged           Aged                                  Alloy Initial  Immediate 2 weeks                                                                              Immediate                                                                             2 weeks                               ______________________________________                                        4     38-43-12 18-26-17  21-29-15                                                                             12-22-21                                                                              13-23-20                              5     33-45-16 21-33-17  24-35-13                                                                             14-30-20                                                                              19-35-19                              6     25-35-15 18-29-18  19-30-18                                                                             13-25-20                                                                              12-26-21                              ______________________________________                                    

EXAMPLE III

In contrast, significantly improved test results were obtained withalloys in accordance with the invention, included in Table IV.

                  TABLE IV (a)                                                    ______________________________________                                        Alloy Mg      Si      Cu     Other       Al                                   ______________________________________                                        B     1.35%   0.64%   1.45%  0.42% Fe    Bal.                                 C     1.00    0.77    1.44   0.42 Fe, 0.38 Mn                                                                          Bal.                                 D     1.41    0.59    1.45   0.18 Cr     Bal.                                 E     1.01    0.67    1.47   0.41 Fe, 0.19 Cr                                                                          Bal.                                 F     1.35    0.74    1.47   0.39 Fe, 0.38 Mn,                                                                         Bal.                                                              0.19 Cr                                          G     0.96    0.76    1.41   0.78 Mn     Bal.                                 H     1.35    0.58    1.41   0.14 Zr     Bal.                                 ______________________________________                                    

                  TABLE IV (b)                                                    ______________________________________                                        Tensile Properties (Y-T-E)                                                             After 10 Secs.                                                                             After 20 Secs.                                                   at 750° F                                                                           at 750° F                                                                 Aged           Aged                                  Alloy Initial  Immediate 2 weeks                                                                              Immediate                                                                             2 weeks                               ______________________________________                                        B     39-53-17 35-45-13  37-48-13                                                                             24-36-13                                                                              31-42-14                              C     48-58-13 37-47-12  36-47-11                                                                             27-39-13                                                                              28-40-12                              D     37-52-16 35-45-14  36-47-16                                                                             27-38-15                                                                              32-44-16                              E     46-57-13 37-47-12  38-47-12                                                                             28-38-13                                                                              28-40-13                              F     44-56-14 34-46-12  35-47-12                                                                             24-39-14                                                                              28-44-14                              G     48-58-13 34-45-12  38-49-13                                                                             23-38-14                                                                              26-41-14                              H     41-53-17 32-41-13  36-45-13                                                                             24-36-14                                                                              29-41-13                              ______________________________________                                    

EXAMPLE IV

Three commercial alloys were selected for direct comparison with alloysin accordance with the invention, yielding test results, as listed inTable V.

                  TABLE V (a)                                                     ______________________________________                                        Alloy  Mg     Si      Cu    Mn    Cr   Others Al                              ______________________________________                                        7 (6351)                                                                             0.5%   1.03%   0.02% 0.49% --   0.38 Fe                                                                              Bal.                            8 (7006)                                                                             2.40   --      --    0.19  0.09 4.53 Zn                                                                              Bal.                            9 (7039)                                                                             2.8    0.072   0.10  0.11  0.17 4.41 Zn                                                                              Bal.                            ______________________________________                                    

                                      TABLE V (b)                                 __________________________________________________________________________              Tensile Properties (Y-T-E)                                                    After 10 Secs. at 750° F                                                            After 20 Secs. at 750° F                        Alloy                                                                              Initial                                                                            Immediate                                                                           Aged 2 weeks                                                                         Immediate                                                                           Aged 2 weeks                                     __________________________________________________________________________    7 (6351)                                                                           38-43-12                                                                           18-26-17                                                                            21-29-15                                                                             12-22-21                                                                            13-23-20                                         8 (7006)                                                                           55-63-12                                                                           21-40-19                                                                            28-50-18                                                                             22-42-21                                                                            30-52-22                                         9 (7039)                                                                           57-65-11                                                                           30-48-16                                                                            29-49-15                                                                             23-45-19                                                                            34-58-18                                         __________________________________________________________________________

EXAMPLE V

Parallel test results listed in Table VI for three alloys in accordancewith the present invention substantiate their significantly superiorresults.

                  TABLE VI (a)                                                    ______________________________________                                        Alloy Mg      Si       Cu    Other       Al                                   ______________________________________                                        J     1.4%    0.64%    1.3%  0.41% Mn    Bal.                                 K     0.95    0.70     1.38  0.41 Mn, 0.21 Cr                                                                          Bal.                                 A     1.38    0.67     1.41  0.39 Mn     Bal.                                 ______________________________________                                    

                  TABLE VI (b)                                                    ______________________________________                                        Tensile Properties (Y-T-E)                                                             After 10 Secs.                                                                             After 20 Secs.                                                   at 750° F                                                                           at 750° F                                                                 Aged           Aged                                  Alloy Initial  Immediate 2 weeks                                                                              Immediate                                                                             2 weeks                               ______________________________________                                        J     43-54-18 34-43-    35-44-15                                                                             24-37-14                                                                              30-44-15                              K     48-58-13 41-50-12  40-51-12                                                                             26-39-13                                                                              28-41-12                              A     41-56-15 33-45-13  37-48-14                                                                             26-39-14                                                                              33-45-14                              ______________________________________                                    

The comparisons afforded by the above two examples show that preferredalloys in accordance with this invention, after low energy welding andnatural aging, are substantially superior to the commercial alloys.After high energy welding and natural aging, the present alloys displayover twice the strength of 6351 and have tensile properties comparableto those of alloys 7006 and 7039, but without their operationaldisadvantages.

EXAMPLE VI

This example substantiates the disadvantageous effects which occur whenthe silicon is present in the alloy in an excess amount, such as to begreater than can be precipitated as a silicide of magnesium or othermetal. The alloys listed in Table VII (a) were prepared as in thepreceding examples and the test results are summarized in Table VII (b),the "Initial" values having been measured on samples prepared at T5temper.

                  TABLE VII (a)                                                   ______________________________________                                        Alloy  Mg      Si      Cu    Mn    Al    Excess Si                            ______________________________________                                        10     0.95%   0.56%   1.46% --    Bal.  0.01%                                11     0.95    0.69    1.4   0.42  Bal.  0.04                                 12     1.00    1.00    1.45  0.44  Bal.  0.31                                 ______________________________________                                    

                  TABLE VII (b)                                                   ______________________________________                                        Tensile Properties (Y-T-E)                                                             After 10 Secs.                                                                             After 20 Secs.                                                   at 750° F                                                                           at 750° F                                                                 Aged           Aged                                  Alloy Initial  Immediate 2 weeks                                                                              Immediate                                                                             2 weeks                               ______________________________________                                        10    46-56-15 37-45-12  37-45-12                                                                             26-37-13                                                                              29-38-12                              11    50-58-13 38-45-10  36-45-12                                                                             27-38-12                                                                              27-39-12                              12    53-60-13 35-44-10  35-43-12                                                                             27-39-12                                                                              28-40-11                              ______________________________________                                    

Thus, the present invention provides aluminum base alloys of highstrength, capable of retaining adequate strength after being subjectedto operations at elevated temperatures, as in fusion welding processes,corresponding to retained yield strength of about 40 ksi or higher forextruded products or somewhat less for hot rolled plate. Strongcrack-free welds are consistently and readily obtainable with thepresent alloys and they show excellent formability for conversion toproducts having good resistance to stress corrosion and other corrosiveinfluences. Accordingly, these alloys are well adapted for use in variedcommercial fields, as in automotive vehicle bodies and components, suchas for tanks and containers.

The above description and specific examples substantiate the attainmentof the specified objectives of this invention in accordance with thealloy compositions and preferred treatment procedures set forth. It willbe understood by those skilled in the art that various modifications mayat times be employed advantageously in the illustrative examples, withinthe scope of the appended claims.

What is claimed is:
 1. An aluminum base alloy of high strengthproperties having improved weldability, consisting essentially of0.9-1.5% magnesium, 0.4-0.8% silicon, 0.9-1.5% copper, and from 0.05 to0.4% of at least one member of the group of elements consisting ofmanganese, iron, and chromium, up to 0.2% each of zirconium, vanadium,and titanium, up to 0.4% cobalt, and up to 3.5% nickel, and balancealuminum, wherein the copper content does not exceed the sum of themagnesium plus silicon contents and the silicon content does not exceedthe sum of 0.58 × magnesium content plus 0.25 × the sum of the manganeseand iron contents, said alloy having substantially equal contents ofmagnesium and of copper.
 2. The alloy of claim 1, wherein the magnesiumcontent is 1-1.5%.
 3. The alloy of claim 1, wherein the copper contentis 1-1.5%.
 4. The alloy of claim 1, wherein the silicon content is0.4-0.7%.
 5. The alloy of claim 1, wherein the alloy contains 0.2-0.4%of at least one member of the group consisting of manganese, iron, andchromium.
 6. The alloy of claim 1, wherein the alloy contains 1.3-1.5%magnesium, 0.6-0.7% silicon, 1.3-1.5% copper, and 0.2-0.4% manganese. 7.The alloy of claim 1, wherein said alloy has been hot rolled and aged.8. The alloy of claim 1, wherein said alloy has been hot and coldrolled, annealed and aged.
 9. A wrought article of high strength, havingimproved weldability, prepared from an aluminum base alloy consistingessentially of 0.9-1.5% magnesium, 0.4-0.8% silicon, 0.9-1.5% copper,and from 0.05 to 0.4% of at least one member of the group of elementsconsisting of manganese, iron, and chromium, up to 0.2% each ofzirconium, vanadium, and titanium, up to 0.4% cobalt, and up to 3.5%nickel, and balance aluminum, wherein the copper content does not exceedthe sum of the magnesium plus silicon contents and the silicon contentdoes not exceed the sum of 0.58 × magnesium content plus 0.25 × the sumof the manganese and iron contents, said article having substantiallyequal contents of magnesium and of copper.
 10. The article of claim 9,wherein said alloy contains 1-1.5% magnesium, 0.4-0.7% silicon, and1-1.5% copper.
 11. The article of claim 9, wherein said alloy contains1.3-1.5% magnesium, 0.6-0.7% silicon, 1.3-1.5% copper, and 0.2-0.4%manganese.
 12. A method for the preparation of wrought products of highstrength properties having improved weldability which comprises:(a)providing an aluminum base alloy consisting essentially of 0.9-1.5%magnesium, 0.4-0.8% silicon, 0.9-1.5% copper, and from 0.05 to 0.4% ofat least one member of the group of elements consisting of manganese,iron, and chromium, up to 0.2% each of zirconium, vanadium, andtitanium, up to 0.4% cobalt, and up to 3.5% nickel, and balancealuminum, wherein the copper content does not exceed the sum of themagnesium plus silicon contents and the silicon content does not exceedthe sum of 0.58 × magnesium content plus 0.25 × the sum of the manganeseand iron contents, said alloy having substantially equal contents ofmagnesium and of copper; (b) casting said alloy; (c) heating said alloyto a homogenizing temperature and thereafter homogenizing said alloy;(d) working said alloy; and (e) aging said alloy, whereby said wroughtproducts are capable of plastic deformation to form articles.
 13. Themethod of claim 12, wherein said alloy contains 1-1.5% magnesium,0.4-0.7% silicon, and 1-1.5% copper.
 14. The method of claim 12, whereinsaid alloy contains 1.3-1.5% magnesium, 0.6-0.7% silicon, 1.3-1.5%copper, and 0.2-0.4% manganese.